
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .one li {
            width: 100px;
            height: 30px;
            line-height: 30px;
            background: url(nav/libg.jpg) no-repeat;
            //float: left;
            text-align: center;
            margin-left: 10px;
            cursor: pointer;
            _display: inline;
            position: relative;
        }
 
        li ul {
            position: absolute;
            left: 0;
            top: 30px;
            display: none;
        }
        .cd{
            margin-top: -30px;
            margin-left: 65px;
        }
        li{
            list-style: none;
        }
    </style>
</head>
<body>
<ul class="one">
    <li id="li01">
        <span>一级菜单</span>
        <ul id="ul01"class="cd" >
            <li>二级菜单</li>
            <li>二级菜单</li>
            <li>二级菜单</li>
        </ul>
    </li>
    <li id="li02">
        <span>一级菜单</span>
        <ul id="ul02" class="cd" >
            <li>二级菜单</li>
            <li>二级菜单</li>
            <li>二级菜单</li>
        </ul>
    </li>
    <li id="li03">
        <span >一级菜单</span>
        <ul id="ul03"class="cd" >
            <li>二级菜单</li>
            <li>二级菜单</li>
            <li>二级菜单</li>
        </ul>
    </li>
</ul>
<script>
    function fn(canshu1,canshu2){
        var li01 = document.getElementById(canshu1);
        var ul01 = document.getElementById(canshu2);
 
        li01.onmouseover = function(){
            ul01.style.display = "block"
        }
        li01.onmouseout = function(){
            ul01.style.display = "none"
        }
    }
    fn("li01","ul01")
    fn("li02","ul02")
    fn("li03","ul03")
</script>
</body>
</html